1. Field of the Invention
The present invention relates generally to the field of electronic devices, and particularly to semiconductor devices.
2. Technical Background
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
A photodiode is a type of diode that is configured to convert light of a particular wavelength into either current or voltage, depending on its mode of operation. The semiconductor materials employed in fabricating a given p-n junction determine the operating wavelength (i.e., color) of light that the diode will convert into current or voltage. In the infrared portion of the spectrum, the wavelength (λ) ranges from approximately 1 to 100 μm. By way of example, a photodiode fabricated using InGaAs would operate at about 1.3 μm, which is in the infrared portion of the spectrum. A photodiode fabricated from AlGaAs would operate in the visible spectrum at about 0.7 μm (red). A photodiode operating at a wavelength of about 0.35 μm (ultraviolet-UV) might be constructed using AlGaN materials. The teachings of the present invention are, therefore, applicable to semiconductor devices operating from the infrared portion of the spectrum to the ultraviolet portion of the spectrum. The present invention should not be necessarily construed as being limited to the material examples provided herein.
The p-n junction is the basic building block employed in many semiconductor devices. Most diodes, including photodiodes and LEDs are p-n junction devices. Certain transistors (e.g., bipolar junction transistors) consist of two back-to-back p-n junctions. Thus, p-n junctions are the basis of semiconductor technologies such as diodes, transistors, integrated circuits and photovoltaic devices. The “p” in the term p-n junction refers to a semiconductor material that has positively charged current carriers (i.e., p-type) and the “n” in the term p-n junction refers to a semiconductor material that has negatively charged current carriers (i.e., n-type). When the two materials are brought together, the p-n junction is formed. In the region where the n-type material is joined to the p-type material, a layer known as the depletion zone is created because the negative electrical charge carriers (electrons) in the n-type material and the positive electrical charge carriers (holes) in the p-type silicon (electrons and holes, respectively) attract and eliminate each other in a process called recombination. By applying a voltage to the p-n junction, the device will provide a flow of electricity in one direction but not in the opposite direction. By way of analogy, diodes (i.e., p-n junctions) are viewed as electrical check valves that are used to turn the flow of electricity ON or OFF. As noted above, a photodiode generates an electrical signal is response to incident light. Energy band diagrams are very often employed as a convenient means for depicting the operation of a p-n junction photodetector.
Referring to FIG. 1, a diagrammatic depiction of a bulk energy band diagram of a conventional photodiode is provided for illustrative purposes. In the horizontal direction, the energy band diagram shows the n-type material 2 at the left (where light is absorbed), depletion region 3 in the middle, and the p-type region 4 at the right. From top-to-bottom, the diagram includes conduction band edge Ec, Fermi energy level EF, and valence band edge Ev. The conduction band edge Ec may be thought of as a region that is almost empty of electrons, and therefore, electrons may freely move in this region. The valence band edge is a region that may be thought of as being almost full of electrons. Thus, electrons cannot freely move in this region. The Fermi energy level marks a border between energy levels. The energy levels below the Fermi level are substantially full of electrons. The energy levels above the Fermi level are substantially empty of electrons. The energy difference 8 between the conduction band Ec and the valence band Ev is known as the energy gap 8, or the bandgap 8, of the semiconductor. The n-type material bends the conduction edge Ec close to the Fermi level EF. In similar fashion, the p-type material bends the valence band edge Ev close to the Fermi level EF. The “bending” of the energy levels in the diagram 1 show that an electric field E is established in the p-n junction 3.
In conductive materials, the conduction band and the valence band overlap, and therefore, there is no energy gap between these layers and electrons may move freely. In insulators, the conduction band and the valence band are widely separated by an energy gap. Thus, the application of even a large amount of energy will typically not generate a current flow. In semiconductors, the energy gap between the conduction edge Ec and the valence edge Ev is smaller. Because the same semiconductor material is used throughout the junction, the energy gap 8 is shown as a constant distance throughout the junction. In a photodiode, when a predetermined amount of light energy 5 is absorbed by the semiconductor, the energy level of electrons in the material is raised above the conduction band Ec and the energy of holes is below the valence band Ev and the device begins to conduct. Thus, when the semiconductor conducts, electrons 7 will move in the conduction band and holes 6 will move in the valence band. In the n-type material, the “majority carriers” are electrons and the “minority carriers” are holes. In the p-type material, the reverse is true; the majority carriers are holes and the minority carriers are electrons. The intrinsic electric field E exerts a force that tends to move holes 6 generated in the absorption region to the right while moving electrons 7 to the left.
When an external electric field is applied to a photodiode, i.e., when the photodiode is “reverse-biased,” the electric field E is greater than the intrinsic electric field and, therefore, the force applied to the electrons 7 and holes 6 is greater. When a photon 5 has an amount of energy greater than the energy gap 8, the energy may excite an electron from the valence band into the conduction band. This creates an electron-hole pair such that electron 7 moves toward the left and hole 6 moves toward the right. At the device level, when the amount of incident light is greater than a predetermined level, electron/hole current will flow through the p-n junction.
One of the issues that detract from the performance of a semiconductor device relates to currents that are not intentionally generated by the device. Clearly, if the device generates extraneous currents, in addition to those generated by photodetection, the sensitivity of the device will be compromised. For example, in certain semiconductors, the energy band gap is relatively small and the introduction of thermal energy can generate a “dark current” in the bulk portion of the device. Another form of dark current is referred to as surface leakage current.
The surface of a compound semiconductor typically has a large density of surface states in a narrow energy range, which pins the surface Fermi level at an energy that is characteristic of the particular semiconductor material. In most large bandgap (EG>1 eV) semiconductors, the surface Fermi level is pinned somewhere in the bandgap, making the surface a semi-insulating depletion layer. Small bandgap semiconductors, such as those used for infrared detectors, often have surface Fermi levels pinned in, or near, one of the bands.
In general, both the magnitude and the type (n-type or p-type) of the conductivity are determined by two factors: the doping for the bulk of a compound semiconductor; and, by the Fermi level pinning for the surface of a semiconductor. The surface conductivity (both type and magnitude) and the bulk conductivity are completely separate phenomenon. For example, regardless of whether the bulk conductivity is p-type or n-type, the surface conductivity type of InAs remains n-type.
In order to fully analyze the bulk and surface currents of a detector, therefore, two energy band diagrams are needed; one energy band diagram is needed for the bulk conductivity and another energy band diagram is required for surface conductivity. The band diagram through the bulk of a conventional photodiode is shown in FIG. 1.
Referring to FIG. 2, a surface energy band diagram of a conventional photodiode having n-type surface conduction is depicted. The energy band diagram shows how surface leakage currents may be generated in photodiodes fabricated using certain semiconductor materials. The Fermi level is shown as being pinned in the conduction band. Thus, the average energy level of the electrons is at an energy state above the conduction band. Because the conduction band may be thought of as a region that is almost empty of electrons, the conventional arrangement exhibits a free path for electrons to flow and unwanted surface leakage current is the result. Those skilled in the art will understand that if the Fermi energy level were shown to be near or below the valence band, a photodiode having p-type surface conduction would be depicted.
Referring to FIG. 3, a perspective view of a conventional InAs photodiode is shown. A common focal plane array (FPA) structure consists of pixels formed of p-n photodiodes processed into mesa structures. Thus, FIG. 3 may be thought of as a single p-n mesa. Bulk currents consist of minority carriers moving across the junction. Bulk current includes diffusion current, tunneling current, photocurrent, and as generation-recombination (g-r) current. The flow of majority carriers across the p-n junction is blocked by its built-in barrier. In the example of FIG. 3, the mesa sidewalls are n-type and have no barriers because of surface Fermi level pinning. As such, the flow of surface leakage current is allowed. Thus, FIG. 3 depicts an “unpassivated” InAs photodiode that suffers from the effects of the surface leakage channel and the resulting free flow of electrons along the surface. Those skilled in the art will understand that in a photodiode having the surface Fermi level at or near the valence band, the mesa sidewalls would be p-type.
FIG. 4 is a diagrammatic depiction of a surface energy band diagram of the conventional InAs photodiode shown in FIG. 1. FIG. 4 is an example illustration of Fermi level pinning in an InAs photodiode. Surface leakage currents can be caused by the pinning of the surface Fermi level or band bending due to stray electric fields at the surface. The latter effect is normally somewhat negligible, however, in small bandgap materials, like those used in long wavelength detectors, and the effects may be significant. As a result, control of surface leakage current is critical to guarantee the best possible device performance. The performance of many types of photodetectors fabricated with multiple types of materials is limited by surface leakage currents. InAs photodetectors typically exhibit surface leakage currents caused by the pinning of the surface Fermi level in the conduction band. This pinning effect results in a surface leakage channel allowing for the free flow of surface electrons, which contributes to the overall dark current and ultimately limits the performance of InAs photodiodes by limiting the specific detectivity.
Heretofore, the discussion has centered on discrete photodiodes. However, photodiodes may be employed in imaging arrays. For example, cooled semiconductor focal plane arrays (FPAs) are the basis of the highest performance infrared imaging technology. FPAs consist of individual pixels fabricated from semiconductor photodetectors, which have low noise and are electrically isolated from neighboring pixels. Surface leakage currents can undermine both of these desired characteristics by adding noise to the individual photodetectors. Surface leakage currents may also create a pixel-to-pixel current path that has a relatively low resistance. The surface leakage currents originate by way of electron states that develop on the surface of air-exposed semiconductor surfaces. Such surface states do not exist in the bulk of the semiconductor, and enable additional conduction paths in parallel to those in the bulk.
Many important types of dark currents, such as diffusion currents and generation-recombination (g-r) currents, are thermally activated processes, which decrease with device temperature. Surface currents, however, are approximately independent of temperature. Many devices can be cooled to decrease diffusion and g-r currents sufficiently so that surface currents become the dominant current, which can then be observed as a temperature-independent current.
In one approach that has been considered, surface passivation treatments are applied to an external portion of the conventional photodiode 1. See FIG. 5. Those of ordinary skill in the art will understand that the application of surface passivation treatments is the conventional approach for controlling surface leakage current. This approach, however, has several drawbacks. For example, the conventional FPA design process can be viewed as occurring in two-steps. First, the epitaxial structure design concentrates on controlling bulk currents and ignores surfaces currents. The aim of such epitaxial designs is to enable the efficient flow of photocurrent and to inhibit (to whatever extent possible) bulk dark currents. Second, attempts to combat surface leakage currents are added as ex-situ processing steps. In other words, the surface passivation treatments are performed after the epitaxial growth, and also after some of the device fabrication steps, whereby a passivation material or treatment is applied to air-exposed surfaces. One disadvantage to this approach relates to the aforementioned additional processing steps that are required. These additional steps add complexity and cost to device manufacturing. Another disadvantage of the ex situ surface passivation treatments is that they are often only partially effective.
What is needed, therefore, is a semiconductor device that substantially eliminates surface leakage currents. What is also needed is a method for in situ manufacturing semiconductor devices that eliminate surface leakage currents. In other words, the method for in situ manufacturing would eliminate the aforementioned second step by fabricating the p-n junction device during epitaxial growth.